The invention relates to the formation of screw threads in mounting holes in furniture hardware made by stamping and pressing from this sheet metal which can be fastened to the furniture by means of a mounting screw threaded through the mounting hole into a bore in a cabinet wall, the shaft of the mounting screw having at its head end an unthreaded section with a diameter smaller than the outside diameter of the thread.
The production of furnitue hardware from sheet metal by the stamping and pressing method permits it to be made cheaply in large series. Such sheet-metal hardware is stronger than hardware made by injection molding methods from plastics or by pressure casting methods from metal, e.g., by die-casting from zinc alloy, and therefore it has a longer life, and, at least in comparison to die-cast metal alloy (Zamak) hardware, it is decidedly cheaper to make. As examples of furniture hardware which are being increasingly stamped from sheet metal and pressed to shape on account of the above-mentioned advantages, we shall mention only mounting plates for furniture hinges, which are mounted on the walls of furniture by means of screws, and on which the carcase-related part of a hinge, today usually in the form of an elongated arm and likewise made mostly from sheet metal by the stamping and pressing method, can be mounted so as to be adjustable in at least two coordinates. Other hardware parts that can be made of sheet metal are, for example, parts of joining hardware, shelf holders and the like. In the industrial installation of hardware--e.g., of the above-mentioned mounting plates--by screwing them to furniture, it is desirable to premount the screws in the hardware such that the hardware together with the premounted and only slightly started screw can be placed in the location where it is to be fastened to the furniture, and then the screws can be driven into the associated bores in the furniture with power screwdrivers. The screws are driven directly into bores in the furniture which are slightly smaller than the major diameter of the threads of the screws and thus create their own female threads in the wall of the bore. To obtain sufficient holding power, screws with relatively coarse threads are used; in recent times so-called "Euro screws" have been used, which have a relatively coarse, sharp-crested thread which can be driven with comparatively light torque into bores in wood materials but have great holding power due to the relatively great depth of penetration of the threads into the wall of the bore, and also they permit the repeated installation and removal of hardware. In hardware of thin sheet metal, the presetting of the mounting screws in the mounting holes, which creates no problems in the case of the relatively thick-walled plastic or die-cast hardware, presents a difficulty, because if they are to be held securely the mounting bore has to be provided with at least one spiral thread into which the thread of the mounting screw can be tightly driven. But the thickness of the sheet metal used is kept as thin as possible, so that the production of a thread running only as little as 270 around the hole within the thickness of the metal is not possible. Often the material thickness of the sheet metal is even less than the pitch length between two successive thread spirals, so that then it is impossible to form a thread spiral capable of securely engaging the thread of the mounting screw. Although the thread could be made in a separate nut which afterwards could be sweated to the hardware, or a cylindrical collar could be created around the mounting hole by forming it from the material of the hardware, and could then be tapped in a second, separate operation, this must be ruled out for reasons of cost, as well as in the case of sheet-metal hardware of low profile, also because the nut or projection protrudes below the bottom of the piece and would prevent it from lying flat on the furniture wall.
The invention is addressed to the problem of making mounting holes in furniture hardware of low profile out of thin sheet metal such that the conventional coarse-thread mounting screws will be able to be preset in the necessary play-free manner in the mounting bore, in correct alignment therewith.